The present invention generally relates to sensors for sensing positions, shifts in positions, rotational speeds and more particularly, relates to wheel speed sensors
An arrangement of this type is disclosed in German patent application No. 196 18 867. In this application, a system for varying a rotational speed signal is described wherein information about the direction of rotation, the air gap, brake lining wear, etc., is superposed on the output signal which prevails in the shape of a binary current or voltage signal.
Further, an active motion sensor with an evaluating circuit is known from German patent application No. 44 34 798. A testing mode is provided in this circuit for testing the xe2x80x98air gap reservexe2x80x99 which is required for compensating dynamic air gap variations or air gap tolerances. In the testing mode, a second hysteresis threshold determines the switch-over of the rotational speed signals during change of the edges of the sensor signal. Switch-over to the hysteresis threshold envisaged for testing is effected, for example, by varying the operating voltage of the sensor. Also, provision is made to switch back to the normal mode as soon as the vehicle wheel reaches a determined speed.
Japanese patent application No. 1065921 discloses the parallel connection of two signal amplifier circuits, one circuit processing a relatively low frequency band, and the parallel circuit processing a higher frequency band. The signal representative of the high input frequency and that one representative of the low input frequency are superposed after amplification in order to achieve a desired frequency characteristics of the amplifier circuit.
The object of the present invention is a sensor arrangement for sensing rotational speeds, positions, shifts in positions, and motional speeds in the field of mechanical engineering, however, more particularly, in the automotive vehicle industry, predominantly in the range of application of controlled brakes, for position indicators, angular position indicators, motive adjustment devices, or for sensing wheel rotational speeds. Sensor arrangements of different types for the above-mentioned purpose are known in the art. Normally, these arrangements comprise a signal generator or pulse generator which is mechanically coupled to the moving machine part, and a sensor or pickup for measuring data which scans the pulse generator. The present invention can be used for arrangements where scanning is carried out by means of light, high frequency, electrical or magnetic fields, especially by means of permanent-magnetic fields. Hole discs, toothed wheels and magnetized structures are known to be used as pulse generators. Especially the magnetoresistive effect (MR elements) or the Hall effect (Hall elements) is used as sensor effects in connection with the present invention.
In German patent application No. 196 34 715 which is later published, an arrangement for sensing the rotational behavior of a rotating encoder is described which includes a sensor module with a sensor element, a controllable current source that furnishes a load-independent current representative of the rotational behavior, and a modulator controlling the current source in response to signals of the sensor element. The sensor module is coupled magnetically to the encoder. The output signal is a signal representative of the rotational behavior with superposed status signals and/or additional signals.
Sensor modules which can resolve an unlimited low rotational speed and those which can resolve a rotational speed which is limited in terms of low values are known in the art. Sensor modules which offer the advantage of an unlimited low rotational speed resolution utilize DC coupled amplifiers and an invariably set amplification, which is restricted by the interfering direct-current portion of the signal and limits the per se possible sensor range to the encoder (air gap). This is a certain shortcoming.
Sensor modules for rotational speeds limited in terms of low values equally utilize DC coupled amplifiers, however, the interfering direct-current portion superposed on the useful signal is compensated by subtraction of a value of equal magnitude so that the useful signal can be amplified to a higher degree.
This is advantageous. For this purpose, the drifting signal direct-current portion is continuously extracted from the signal by a low-pass filter function of a low limit frequency and countercoupled to the signal so that a high-pass behavior is achieved at the output which limits the bottom rotational speed that can be sensed. This is a disadvantage in brake applications where the objective is to react from standstill, or when the standstill must be known for calibration purposes.
An object of the present invention is to provide an arrangement for sensing rotational speeds or measuring rotational speeds which furnishes precise, reliable and undisturbed measuring results, even under unfavorable conditions such as great air gap tolerances and dynamic air slot variations, over the wide rotational speed and/or frequency range that must be covered, for example, in an automotive vehicle control system (ABS, TCS, ESP, etc.).
It has been found that this object is achieved by the present invention arrangement of the type mentioned in the preamble of claim 1, including that at least the sensor element, the controllable current source and a part of the signal-conditioning circuit and the modulator are integrated in a sensor module, that the modulator controls the current source in response to the signals of the sensor element and external signals, and that the signal-conditioning circuit has at least two different signal-conditioning networks which, by way of a control network and in dependence on predetermined criteria and/or conditions, are connected between the sensor element and the modulator and take care of signal conditioning.